The invention relates generally to a method and apparatus for adjusting color saturation in an electronic image processing system, in which the original images are electro-optically scanned in three primary colors by rows and columns, and, more particularly, to a method and apparatus in which the image signals are transformed to a luminance signal Y and two color difference signals or chrominance signals C1, C2.
In the reproduction of colored original images (positive-positive) or the manufacture of color photographic positive pictures from color negative originals, greater and greater reliance is being placed on electronic image processing and the use of color correction circuits. See, for example, European Patent Applications Nos. 70 680, 131 430, and 168 818; European Patent Application No. 168 818 corresponds to U.S. Pat. No. 4,661,843.
The fundamentals of electronic color correction are described in, for example, the books Farbmetrick und Farbfernsehen (Color Measurement and Color Television), by H. Lang, pages 326 to 334 and 431ff, R. Oldenbourg-Verlag publishing house, Munich/Vienna 1978, and Digital Image Processing, by W. K. Pratt, particularly pages 50-90 and 155-161, John Wiley & Sons, New York/Chicester/Brisbane/Toronto, 1978.
Transformation into luminance and chrominance signals is known in principle in the video art. There it is directed primarily to using electronic means to undertake color corrections if reception-induced color errors are present (e.g. color distortion) or if, during transmission, false color casts arise, which must be compensated.
Experience has shown that, for production of optimal positive images, e.g. on color copier material, the following image parameters must be adjusted or be capable of being adjusted:
(a) color balance, PA1 (b) color saturation, PA1 (c) global contrast (gradation).
There is often the difficulty that these parameters cannot be adjusted independently of one another. When one adjusts the gradation, one finds that, for example, the color saturation has changed. Nevertheless, an independent adjustment can be carried out when the brightness-linear image signals are transformed into luminance and chrominance signals, as is usual in the television art (the so-called RGB-to-YUV transformation).
The desired adjustment of the color saturation can be done continuously with a potentiometer or in steps using individual keys. It has been found that, in order for the images to be classified as optimal by an observer, the production of color photographic prints from color negatives or color positives must satisfy special criteria, some of which relate to physiology. In particular, it has been shown that the adjustment of color saturation becomes critical when weakly saturated regions are present next to highly saturated regions in the original image. In practice, the color channels for the chrominance signals (color difference signals) have a limited modulation range, so that the case can arise that increasing the color saturation in one or both channels causes the modulation limit to be reached or exceeded. This over-modulation leads to an undesired change in coloration, i.e. to false coloration.